Security Articles from Wikipedia

Digital signature 47
Digital signatures vs. ink on paper signatures
An ink signature could be replicated from one document to another by copying the image manually or digitally, but
to have credible signature copies that can resist some scrutiny is a significant manual or technical skill, and to
produce ink signature copies that resist professional scrutiny is very difficult.
Digital signatures cryptographically bind an electronic identity to an electronic document and the digital signature
cannot be copied to another document. Paper contracts sometimes have the ink signature block on the last page, and
the previous pages may be replaced after a signature is applied. Digital signatures can be applied to an entire
document, such that the digital signature on the last page will indicate tampering if any data on any of the pages have
been altered, but this can also be achieved by signing with ink all pages of the contract.
Additionally, most digital certificates provided by certificate authorities to end users to sign documents can be
obtained by at most gaining access to a victim's email inbox.
Important paper documents are signed in ink with all involved parties meeting in person, with additional
identification forms other than the actual presence (like driver's licence, passports, fingerprints, etc.), and most
usually with the presence of a respected notary that knows the involved parties, the signing often happens in a
building which has security cameras and other forms of identification and physical security. The security that is
added by these type of ink on paper signatures cannot be currently matched by digital only signatures.
Some digital signature algorithms
• RSA-based signature schemes, such as RSA-PSS
• DSA and its elliptic curve variant ECDSA
• ElGamal signature scheme as the predecessor to DSA, and variants Schnorr signature and Pointcheval–Stern
signature algorithm
• Rabin signature algorithm
• Pairing-based schemes such as BLS
• Undeniable signatures
• Aggregate signature - a signature scheme that supports aggregation: Given n signatures on n messages from n
users, it is possible to aggregate all these signatures into a single signature whose size is constant in the number of
users. This single signature will convince the verifier that the n users did indeed sign the n original messages.
The current state of use — legal and practical
Digital signature schemes share basic prerequisites that— regardless of cryptographic theory or legal provision—
they need to have meaning:
Quality algorithms
Some public-key algorithms are known to be insecure, practicable attacks against them having been
discovered.
Quality implementations
An implementation of a good algorithm (or protocol) with mistake(s) will not work.
The private key must remain private
if it becomes known to any other party, that party can produce perfect digital signatures of anything
whatsoever.
The public key owner must be verifiable
A public key associated with Bob actually came from Bob. This is commonly done using a public key
infrastructure and the public key user association is attested by the operator of the PKI (called a certificate